For Long Term Evolution (LTE) Release 8 (R8) and Release 9 (R9), wireless transmit/receive units (WTRUs) transmit sounding reference signals (SRS) periodically based on a schedule and transmission parameters that are provided semi-statically to the WTRU by the evolved Node B (eNB) via a combination of broadcast and radio resource control (RRC) dedicated signaling. Cell-specific SRS configurations define the subframes in which SRS are permitted to be transmitted by WTRUs for a given cell. WTRU-specific SRS configurations define the subframes and the transmission parameters to be used by a specific WTRU. These configurations are provided to the WTRU via RRC signaling. In its WTRU-specific subframes, a WTRU may transmit SRS in the last symbol across the entire frequency band of interest with a single SRS transmission, or across part of the band with hopping in the frequency domain in such a way that a sequence of SRS transmissions jointly covers the frequency band of interest. The cyclic shift and the transmission comb are configurable by higher layer signaling. In LTE R8/9, a maximum of eight different cyclic shifts are possible and two different transmission combs. The transmission comb is a frequency multiplexing scheme; each comb includes every other subcarrier over the band of interest. In contrast to the multiplexing of SRS transmission by means of different cyclic shifts, frequency multiplexing of SRS transmissions does not require the transmissions to cover identical frequency bands.
LTE-Advanced (LTE-A), (referring to at least LTE Release 10 (LTE R10)), may provide aperiodic SRS transmissions to reduce the number of unnecessary SRS transmissions and to alleviate the potential problem of not having enough SRS resources to support the added SRS transmissions needed for WTRUs with multiple antennas. In particular, dynamic aperiodic SRS may be provided but signaling and other aspects have not been identified. For aperiodic SRS transmission, a WTRU may need to know in what subframe(s) to transmit the SRS and with what parameters. In addition to the LTE R8 parameters, such as cyclic shift and transmission comb, the WTRU may also need to know on which component carrier (CC) and with which antenna(s) to transmit. In order for the WTRU to know when to transmit the aperiodic SRS, several triggering mechanisms may be used including uplink (UL) grants, downlink (DL) grants, RRC signaling, medium access control (MAC) control elements and group-based or individual-based physical downlink control channels (PDCCH). With respect to the use of UL or DL grants, activation bit(s) may be used as well as having the grant alone be the trigger but no particulars have been provided. Mechanisms for configuring the SRS transmission resources/parameters may include semi-static configuration via RRC signaling as well as PDCCH based configuration being included with the trigger but again no particulars have been provided.